Near edge X-ray absorption fine structure resonances of quinoide molecules

We report on C 1s and N 1s near-edge X-ray absorption (NEXAFS) investigations of highly oriented ultrathin films of 2,5-dimethyl-N,N'-dicyanoquinonediimine (DMe-DCNQI), tetracyano-quinonedimethane (TCNQ), and 2,5-dimethyl-1,4-benzoquinone (DMe-BQ) adsorbed on Ag(111). Several sharp resonances are resolved in the spectra indicating the localized electronic structure of these quinoide molecules. The comparison of the angle-dependent DMe-DCNQI results with those from other molecules and literature data on cyano-containing molecules leads to the conclusion that the common building-block approach is not sufficient, but that the interpretation of most valence orbitals being delocalized within the whole molecule appears to be more appropriate as confirmed by semiempirical INDO/S SDCI. calculations. The NEXAFS spectra of the related DMe-DCNQI charge transfer salts also reflect strongly delocalized molecular orbitals very different from those of unreacted DCNQI.